This invention relates to a method for producing a protective coating on materials such as aluminum, as well as to a coating produced by the method. The invention is in particular directed to a method and coating produced thereby, employing zirconia.
Aluminum is used extensively in industry. While the application of protective coatings to aluminum to enhance its usefulness is known, the application of ceramic coatings to low melting temperature materials such as aluminum has not been considered practical since such materials generally require thermal processes that would result in weakening the substrate, even though they may impart desirable surface properties that would extend the life and improve the efficiency of the aluminum component. Thus, in many instances coated aluminum could economically replace heavier metals, if properly protected. While many applications exist for such coatings, existing coatings either don't effectively protect the aluminum or other material, or require processing temperature that disadvantageously affect the aluminum.
The use of zirconia has been suggested in the past for various coatings, and as an additive. Thus, U.S. Pat. No. 3,875,971, Hamling, discloses the use of a zirconia coating, wherein an acidic zirconia coating is applied to a porcelain enamel coating on a metal. U.S. Pat. No. 4,624,831, Tommis, discloses the addition of zirconia fibers directly to molten aluminum to produce a composition with a melting point higher than aluminum. U.S. Pat. No. 3,632,359, Alper, discloses the addition of zirconia to a cast alumina-silicon refractory for the glass contact lining of a furnace, to decrease the tendency of the refractory to crack. U.S. Pat. No. 3,754,978, Elmer, discloses a glaze for glass from a slurry of water, powdered alumina and powdered zirconia, with an addition of ammonia to give a pH of 8.5. The slurry is dried on the glass with a flame at about 650.degree. C., and finally reacted in a gas flame to produce a vitreous layer. U.S. Pat. No. 3,899,341, Schwarz discloses a refractory fired shaped element of zirconia oxide and zirconium silicate, the element being cast in gypsum molds and fired at about 1650.degree. C. U.S. Pat. No. 4,585,499, Mase, discloses a ceramic material formed of a slurry of zirconia powder and a non-aqueous solvent, the product being fired at a temperature above 1,100.degree. C. U.S. Pat. No. 4,621,064, Matsuura, discloses a low temperature sealing material, for example for sealing integrated circuit packages, of powdered glass, zinc oxide, silica and aluminum powder, and from 1 to 35% zirconia powder. U.S. Pat. No. 2,061,099, Morgan, discloses a refractory material encorporating zirconia, and adapted to be heat treated at temperatures from 600.degree. to 1800.degree. F. U.S. Pat. No. 4,544,607, Nagoya, discloses a ceramic composition encorporating zirconia, for use in an engine.
U.S. Pat. No. 3,285,757, Cornely discloses a cement composition useful for making bonds or castings, in which a compound is provided which includes a zirconium compound such as zirconia, and a binder precursor compound such as water soluble silicate. The sodium silicate is at least 8% by weight, and preferably at least 25%, of the combined weights of zirconium compounds that are used. In the aqueous solution as used, the silicate is about 26-32% by weight of the solution. A thin coating is applied to the pieces to be joined, they are joined together, and the cement is allowed to air dry. While the drying time may be overnight at room temperature, or at 160 to 170 degrees Fahrenheit for one hour, Cornely requires a high curing temperature, for example at 1100 degrees Fahrenheit for 20 minutes, to effect a final chemical action, at the high temperature, between highly viscous silicate and the zirconia and zircon.
The process of densification of a porous ceramic surface is known. In known techniques, however, curing temperatures of at least 600 degrees Fahrenheit have been required in order to convert chromium compounds in the densification solution to water insoluble chromium oxide. Thus, U.S. Pat. Nos. 3,734,767; 3,789,096; 3,817,781; 3,925,575: 3,944,683; 4,007,020; and 4,077,808, Church et al, disclose the densification of a ceramic by repeated steps of impregnating the ceramic with a metal capable of being converted to an oxide in situ, at temperatures of at least 600 degrees Fahrenheit. U.S. Pat. No. 3,873,344, Church et al discloses the densification of porous underfired ceramics, for use as bearing materials, wherein the ceramic is impregnated with a solution of a chromium compound and cured in one or more cure cycles of at least 600 degrees Fahrenheit, at least one cure cycle being at 1,300 degrees Fahrenheit. U.S. Pat. No. 3,956,531, Church et al discloses the densification of porous ceramic bodies by impregnating with a solution of chromium oxide and curing at temperatures in excess of 600 degrees Fahrenheit. U.S. Pat. No. 3,985,916, Church et al discloses the densification of metal parts plated with porous chrome with a chromic acid solution, the product being cured at a temperature of at least 600 degrees Fahrenheit. U.S. Pat. No. 4,102,085, Church et al discloses a process for producing an abrasive surface wherein a coating of an abrasive, a ductile metal powder and a binder of a soluble chromic compound is applied to an oxide coating on a metal substrate, and cured at a temperature of at least 600 degrees Fahrenheit. The process may be repeated. U.S. Pat. No. 4,615,913, Jones et al discloses a method for providing a thicker coating, employing chromium compound densification, and also requiring curing at a temperature of at least 600 degrees Fahrenheit to convert the chromium compound to a water insoluble chromium oxide.